Electro-acoustic systems are in common use in a variety of forms, most commonly in home stereo systems. These electro-acoustic systems receive an electrical input, for example, from a compact disc (CD) player or a tape deck, amplify the input signal significantly, and then apply it to two or more acoustic transducers, e.g., loud speakers. Although the performance of such systems is often judged quite subjectively, there are a number of objective performance parameters associated with electro-acoustic systems. The most important of these parameters is the frequency response of the electro-acoustic system, both in terms of its bandwidth between low and high cutoff frequencies and the degree of amplitude variation between those cutoff frequencies.
The frequency response of an electro-acoustic system is typically measured by applying a stimulus signal to the electrical input of the system, and picking up the resulting acoustic signal with a calibrated microphone. The microphone output signal is then examined to determine the frequency response of the electro-acoustic system. The frequency response can be measured in either the time domain or the frequency domain. The frequency response is generally measured in the frequency domain by applying a constant amplitude, swept frequency sign wave to the input of the system, and measuring the amplitude of the microphone output signal. The frequency of the input signal is generally plotted along the X-axis of a display while the intensity of the amplitude of the microphone output signal is plotted along the Y-axis. Frequency response can be measured in the time domain by applying a stimulus pulse to the input of the system, and then performing a fast Fourier transform on the resulting pulse at the output of the microphone.
Regardless of whether the frequency response of an electro-acoustic system is measured in the time domain or the frequency domain, the results are less than optimum. The primary limitation on either approach is the subjective manner in which the high and low cutoff frequencies are identified. In theory, the high and low cutoff frequencies are the frequencies at which the amplitude of the transfer function from the output of the system to its input falls 3 dB from the presumably flat amplitude between the cutoff frequencies. However, there are two fallacies to this approach. First, the transfer function of the electro-acoustic signal is not exactly flat between the upper and lower cutoff frequencies. Thus, there is often no clear 0 dB point that can be used as a reference to determine when the transfer function is 3 dB down from the reference point. Second, the conventional approach assumes that the transfer function rolls off smoothly at the high and low cutoff frequencies. In reality, the transfer function is normally composed of a series of peaks and troughs created by imperfections in the acoustic transducers which often make the frequency at which the transfer function is "3 dB down" impossible to determine accurately. Thus, under many circumstances, a subjective guess is made to determine the bandwidth of the electro-acoustic system. Furthermore, measuring the bandwidth of an electro-acoustic system using the conventional approach is quite time-consuming, and to achieve even fairly accurate results, it must be performed by a fairly skilled technician.
Another important performance parameter of an electro-acoustic system is its thermal limit. Acoustic transducers, such as loud speakers, are generally rated by their manufacturers as being capable of handling a specified power. However, well before this power limit is reached, the voice coil of the transducer become quite hot. As the temperature of the coil increases, the impedance of the coil markedly increases, thus limiting the power that is being applied to the acoustic transducer. Accurate data specifying efficiency loss resulting from voice coil heating is generally not specified by the manufacturer, and there does not seem to be any standard relationship between the power capabilities of the transducer and the power at which efficiency decreases. Thus, under most circumstances, it is not possible to determine the acoustic power that a transducer is actually capable of delivering. The problem becomes even more acute when different transducers in a multi-transducer array reach their thermal limits at different applied powers. Under these circumstances, the multi-transducer array performs in one manner at relatively low applied power and performs in an entirely different manner at significantly higher powers when some of the transducers in the array have reached their thermal limits. Under these circumstances, a variety of dynamic frequency response aberrations and polar shifts can occur.
Another critical performance parameter of electro-acoustic systems is group delay which can be useful in identifying dips that can be corrected through equalization. In a multi-transducer array, it is usually assumed that the transducers behave in the same manner and thus act as one large transducer. In reality, since the transducers are spaced apart from each other, nulls occur as the acoustic signals from each of the transducers interact constructively and destructively. These nulls cannot be corrected by simply applying more power to the acoustic transducer at the null frequency through equalization. Other localized amplitude reductions are not caused by interference between two or more acoustic transducers. These amplitude reductions, known as "dips," are correctable through equalization. It is important to be able to differentiate between equalizable dips and unequalizable nulls because attempting to correct unequalizable nulls by simply pumping more power into the acoustic transducer can cause damage and degrade performance. Equalizable dips are amplitude reductions in which the amplitude reduction is accompanied by phase shifts between the input and output of the system that are substantially the same at frequencies below and above the frequency of the dip. In other words, a dip is equalizable if the phase shift between the output and input of the system varies at the dip frequency but is the same at frequencies below and above the dip frequency. If, however, the phase shift between the input and output of the electro-acoustic system shifts from one value below the frequency of the amplitude reduction to a substantially different value above that frequency, a null exists that cannot be corrected through equalization. The difficulty in determining the phase shift and related group delay parameter of electro-acoustic systems has limited the ability to differentiate between correctable dips and incorrectable nulls in electro-acoustic systems.
Still another performance parameter of electro-acoustic systems is spurious vibrations that may be generated by either the electro-acoustic system itself or the environment in which the electro-acoustic system is installed. Spurious vibrations are characterized as vibrations at a frequency other than the frequency of the acoustic signal. For example, a strong acoustic signal at one frequency may cause walls, door panels, glass panels or any other type of mechano-acoustic narrow band absorber to vibrate at the resonant frequency of the absorber. It can often be very difficult to diagnose and correct these spurious vibrations because they are often intermittent and occur at only specific frequencies which may be present only momentarily in a musical work. As a result, it has been extremely difficult and time-consuming to identify the causes of spurious vibrations and to correct those vibrations once their sources are identified.
In summary, while the above described performance parameters in electro-acoustic systems have been analyzed by skilled technicians using sophisticated laboratory equipment to perform time-consuming tests, there has heretofore not been any device that is capable of quickly and easily analyzing a variety of electro-acoustic performance parameters by relatively untrained personnel.